1. Field of the Invention
The present invention relates to apparatus and method for compensating the distortion on a screen display, and particularly to apparatus and method for compensating the distortion on the bottom and top edges of a screen display.
2. Description of the Prior Art
A display on a monitor or a television set is usually distorted geometrically due to the imperfection of a cathode-ray tube or the influence from the surrounding environment. FIGS. 1A to 1F demonstrate some types of geometrical distortion, which are often referred to as horizontally trapezoidal distortion. This distortion exists on the right or left edge, and is caused mainly by the imperfection of the yoke surrounding a cathode-ray tube (CRT) during manufacturing. Although the supply of the CRTs is regulated by CRT manufacturers according to their specification, the distortion is quite noticeable for CRT larger than 17 inches. Therefore, monitor manufacturers usually add some compensating circuits to correct this type of distortion.
FIGS. 2A to 2D show another type of geometrical distortion, which are usually referred to as horizontal pincushion distortion. This distortion exists on the right or left edge, and is caused mainly by the non-roundness on the surface of the screen. The monitor manufacturers usually compensate this type of distortion through the adjustment of a bias current in the monitor. For example, the distortion in FIG. 2C can be corrected by making the length of the central horizontal scan longer than the length of the top or the bottom scan.
FIGS. 3A to 3D further show some examples of pincushion type of geometrical distortion. These examples differ from those in FIGS. 2A-2D on the locations where the distortion occurs. The pincushion type of distortion on the top or bottom edge is also caused by the non-roundness on the surface of the scan, and is usually corrected through the yoke by the CRT manufacturer. Unfortunately, for the pincushion type of distortion on the top or bottom edge, the monitor manufactures usually do not apply further compensation or adjustment unless excellent quality requested.
FIGS. 4A 4F demonstrate another trapezoidal distortion on the top and/or bottom edge. Although the supply of the CRTs is regulated by CRT manufacturers according to their specification, this type of distortion is quite noticeable for CRT larger than 17 inches. Unfortunately, their is no apparatus or method provided for compensating the distortion on the bottom and top edges of a screen display.
A rotate coil 52, shown in FIG. 5, surrounding a cathode-ray tube 50 and near a yoke 54, is used to correct a tilt problem of the screen display. However, this rotate coil 52 could not be used directly to solve any aforementioned type of distortion.